Various glass-like polysaccharides are known in the art, including those disclosed in U.S. Pat. No. 3,706,598 and AU 74083/87. According to the Australian patent, such products have three basic applications, namely: snack foods, breakfast cereals and instant products. Such products are produced with a view to providing an intermediate that is readily puffed into a snack food or cereal, or in the production of foods such as instant pasta that can be rapidly hydrated on cooking (i.e. the structure is readily permeated by water during heating).
The U.S. patent relates to the production of glass-like starch hydrolysates having a dextrose equivalent of between 10 and 25, which are useful in dehydrated foods as bulking agents/humectants or as additives to soups or sauces, and which are readily dispersible in water.
In both cases the desired properties are related to the production of glass-like starches in which the internal structure of the product is sufficiently under-developed or disrupted as to allow water to be rapidly absorbed on the one hand, or rapidly released on the other.
Various abrasive grit materials are well known in a variety of arts, and have traditionally found application as sand blasting abrasives, most typically in applications such as cleaning building exteriors, or removing surface oxides from castings. Examples of other applications include those described in U.S. Pat. No. 3,485,671, which pertains to the scouring of the interiors of central air supply duct work and plenums. In such applications, the minor damage done to the surface being treated is not usually of any moment.
Applications exist, however, wherein it would be advantageous to utilize abrasive grits, but wherein the use of conventional grit materials is not acceptable because of consequential surface damage associated with their use. These include, for example, such applications as those disclosed in U.S. Pat. No. 4,659,391 which teaches using relatively soft grit materials entrained in a low velocity air stream in order to clean and remove flashing from the surfaces of molded sand cores, i.e. unfinished metallic castings having complex surfaces such as holes, projections, notches, and/or other irregularly shaped surfaces that are readily damaged by more traditional sand blasting media. Preferred particles are formed of polyethylene or polycarbonate cylinders having a length and a diameter in the size range of about 0.06 inches.
Another application involving sensitive substrates is described in U.S. Pat. No. 4,545,155. In this patent, abrasive grit material is used to remove flashing from molded electronic casings. In this application the generation of static electricity by the abrasive is also a problem. The patent teaches using a slurry comprised of a synthetic resin which is dispersed in water containing a surfactant. Examples of abrasive materials include thermosetting resins such as an epoxy resin, a urea resin, an unsaturated polyester resin, an alkyd resin, or relatively harder resins such as polystyrene, polycarbonate or acrylic. In this application a grit hardness equal to or close to that of the substrate is desired. These grits are preferred over those used previously in this application, such as alumina, silicon carbide, or glass beads, or even relatively softer abrasives such as walnut shell powder, because even the softest of these abrasives is much harder than the casings substrate, (typically epoxy resin), and therefore results in unacceptable surface damage.
U.S. Pat. Nos. 3,090,166 and 3,142,590 each describe the use of abrasives in various polishing applications where a superior surface finish is desired. An example of one especially sensitive application pertains to the commercial and military aircraft industries, wherein the paint on the aircraft's exterior must be removed periodically, without occasioning damage to the underlying surfaces or surface finishes.
The problem is made especially acute where the aircraft's skin is coated with an aluminum cladding. Aluminum cladding is generally not less than 4% of the coated sheets thickness for sheets that are less than 0.064 inches thick, and not more than 2% of the thickness of the sheet, for sheets that ar more than 0.064 inches thick. Damage to the coating can necessitate the removal of whole sections of the aircraft's skin for replacement.
Other surfaces of the aircraft may be of anodized or composite materials (i.e. fiberglass) which are also delicate.
Until relatively recently, therefore, the airline industries have stripped aircraft paint using methylene chloride based chemical paint removers. More stringent environmental legislation and health standards have made this approach both inconvenient and prohibitively expensive. A number of companies have actually had to postpone aircraft exterior surface maintenance programs rather than attempt to deal with the regulations and costs associated with chemical stripping.
A variety of alternative approaches using non-chemical, dry stripping of such paint surfaces have been attempted. These include the use of sugar, and other fine particulate matter. With regard to the use of sugar, experience has shown it to be too fine to be effective in its normal commercially available form. Solubilitization of commercially available crystalline sugar in water, followed by recrystallization to produce larger crystal structures does not improve its performance. Also sugar crystals are to brittle and hence are amenable to only a single use, which contributes greatly to its cost in this application.
A number of other organic materials have been employed as abrasive grits. U.S. Pat. No. 3,424,616, for example, describes the use of comminuted and pulverized vegetable material, preferably a cellulose material such as corncob grits, rice hulls. soybean hulls, and tapioca. U.S. Pat. No. 2,622.047 describes the use of lignocellulosic materials having densities of at least 1.2 and a size in the range of 12 to 40 mesh. Suitable materials are disclosed as including nut shells and seed stones such as shells of pecans, almonds, black walnut, English walnuts, and apricot, peach, or olive pits and the like. In general hulls from nut meats, although soft relative to more traditional grit materials, are still too hard for epoxy surfaces, according to U.S. Pat. No. 4,545,155. In addition, ground walnut shells disintegrate on contact with the surface of the aircraft, producing a fine dust and rendering the material substantially useless for any subsequent reuse in this application. Moreover, the dust represents an unacceptable explosion hazard and these materials are not used commercially for this reason. These problems are substantially the same as those which are dealt with in CA1094332, mentioned supra.
U.S. Pat. No. 2,426,072 discloses that it is known to use substances such as cracked wheat, clover seed, and other grains. This same patent, however, also teaches that such grits are, by and large unsatisfactory, and subject to numerous and significant disadvantages. For the reasons set out in this patent, synthetic resins are taught to have substantially superior qualities over the cereal grits. This is a view which still persists in the art today and engineering plastic materials continue to be used in commercial applications, most notably in aircraft paint stripping applications.
In spite of their advantages, the use of engineering plastic materials in grit applications still requires trained personnel, since the grits are sufficiently abrasive to result in permanent damage to aircraft skins, and require careful, skilled handling. Part replacement arising as a consequence of unacceptable amounts of surface abrasion are not uncommon. Notwithstanding the fact that engineering plastic grits are an improvement over more conventional materials with respect to the amount of dust generated in the course of their use, the use of these engineering plastic grits still requires dedication of facilities to dry-stripping operations, including the installation of large volume ventilation systems to avoid health and explosion problems created by the dust that is generated. In addition, removal of a portion, and rearrangement of the balance, of the aluminum-clad surface coating that results from the use of plastic grit can yield an unpolishable surface and can even result in increased surface area on the exterior surface and material stresses within the aircraft's skin.
Attempts at overcoming some of these shortcomings in the use of engineering plastic grit materials include those described in U.S. Pat. No. 4,731,125 which relates to the use of a special soft grit material at low operating pressures. Particles of plastic material such as urea formaldehyde or another thermoset resin are suggested. These particles have an apparent hardness of less than about 3.5 moh and are utilized in a fluid stream operating at about 40 psi.
In the cleaning of used PCB containers, such as barrels for example, the abrasive grits may serve to clean the interior of the containers.
There is a continuing need for inexpensive grit materials that are not heavily dependent on petroleum stock reserves and are preferably biodegradable.